The nature of topological parameters. I. Are topological parameters 'fundamental properties'?

Abstract

Topological parameters are often used descriptors in QSAR and QSPR studies. Though easily generated from an appropriate algorithm useful in developing predictive models they are not readily interpretable. Though every molecule has an intrinsic topology it is not clear whether topological parameters are 'fundamental' properties or not. The results of correlations of various properties of sets of molecules with constant topology shows that are a function of the polarizability a alone in the case of nonpolar molecules and of both dipole moment mu and polarizability alpha in the case of polar molecules. Properties studied include melting point, boiling point, critical temperature, heat of melting, heat of vaporization, solubilties of gases in various solvents, and the ratio of the Van der Waals constants (a/b). Types of molecules studied include monatomic (rare gases), diatomic, trigonal pyramidal, and tetrahedral. Our results show that these properties are successfully modeled by alpha and mu. As they are isotopological in each data set they cannot be modeled by topological parameters. It follows then that topological parameters do not have a direct cause and effect relationship with these properties and therefore are not fundamental parameters. The conclusion of Kier and Hall that they represent electron accessibility is incorrect. Topological parameters are composite parameters representing counts of the numbers of atoms, bonds, electrons, and branching.